1. Field of the Invention
This invention relates to implantable cardioverting and defibrillating pacemakers. More particularly it relates to an apparatus that adds the ability to transduce haemodynamic compromise to a cardioverting/defibrillating pacemaker.
2. Background Art
Pacemakers were initially developed to electrically stimulate hearts that were unable to beat at a rate sufficient to maintain a life sustaining cardiac output. The first devices delivered electrical stimuli at a fixed rate regardless of the heart's function or the body's physiological needs.
Devices were then developed that stimulated the heart only if it failed to beat above a predetermined rate. Such devices sensed the electrical activity of the heart, usually in the right ventricle. Later developments saw the introduction of pacemakers that sensed and stimulated in both the right atrium and the right ventricle.
Pacemakers were also introduced that obtain a measure of the body's physiological need and that responded by altering the paced rate to meet the demand, for example, by sensing the respiratory rate and then increasing the heart rate as the respiratory rate increased. Such a pacemaker is disclosed in U.S. Pat. No. 4,702,253 to Nappholz et al.
Devices were developed that electrically sensed the presence of a ventricular tachyarrhythmia and delivered a defibrillating D.C. shock to revert the heart to a normal rhythm. More advanced devices were developed that attempted to pace hearts undergoing a supraventricular or ventricular tachyarrhythmia back into a normal rhythm. This technique is known as antitachycardia pacing.
Devices have been developed that can act both as pacemakers and as arrhythmia control systems. These devices are able to pace a heart that is beating too slowly, to cardiovert/defibrillate a heart, and to pace a heart undergoing a ventricular tachyarrhythmia, back into a normal rhythm. The Guardian device is such a device and is described in U.S. patent application Ser. No. 187,797, of R. Grevis and N. Gilli, filed Apr. 29, 1988, and entitled "Apparatus and Method for Controlling Multiple Sensitivities in Arrhythmia Control Systems Including Post-therapy Pacing Delay", now U.S. Pat. No. 4,940,054.
The Guardian device is a microcomputer based arrhythmia control system. It is able to be programmed to many different bradycardia pacing modes. A telemetric link is used to communicate with the physician. Variables such as the bradycardia support pacing rate and the atrio-ventricular (AV) delay can be programmed to suit the needs of the recipient of the device. However, such parameters can only be altered by a telemetric link. There is no provision for the device to adjust its programmed parameters in a learning response mode.
The use of a telemetric link allows not only the reprogramming of a device, but also the interrogation of a device by a clinician. Some devices are also fitted with vibrating warning devices to indicate to the patient certain error states of the device and/or malfunctions of the heart. The idea is to hasten the patient's presentation to the clinician to allow interrogation of the device.
Despite the above developments, there are still some limitations inherent in any device that relies solely upon the sensing of the electrical activity of the heart as its means of determining the state of cardiac function. Such devices can be confused by electrical noise induced in the sensing circuits and have difficulty distinguishing a supraventricular from a ventricular tachyarrhythmia. Furthermore they are not able to determine whether or not a tachyarrhythmia is haemodynamically compromising, regardless of its origin. There are differences in the haemodynamic effects of the different tachyarrhythmias as documented by Nakano in his article "Effects of Atrial and Ventricular Tachycardias on the Cardiovascular System." Am. J. Physiol 206: 547-552 (1964).
The result of these shortcomings is that recipients of cardioverter/defibrillators and cardioverting/defibrillating pacemakers may be subject to the inappropriate delivery of defibrillation therapy. Such therapy is not without risk of damage to the myocardium. Furthermore, unwarranted discharge of the device causes pain to the conscious patient, instilling great anxiety, as well as shortening the life of the batteries that power the device.
Haemodynamic compromise exists when there is either insufficient blood pressure or blood flow to meet the oxygen demands of the tissues of the body (See Guyton A., "Textbook of Medical Physiology", 7th Ed., Saunders 1986). It is a relative term since the amount of oxygen required varies with the level of activity, the level of consciousness, feeding etc.
Monitoring the blood pressure is an effective means, commonly used in clinical practice, to assess an individual's haemodynamic state. A fall in arterial blood pressure is associated first with a loss of consciousness, then with ischaemia of vital organs, and finally with death either acutely due to anoxic brain death or, in the longer term, with the failure of other vital organs.
The heart is a cyclical pump with a pulsatile output that is smoothed in the capacitance vessels to produce a steady capillary flow of oxygen rich blood to the tissues. Thus arterial blood pressure shows cyclical peaks and troughs; the systolic and diastolic pressures.
The ventricular pressure, likewise, cyclically increases and decreases and is a measure of an individual's haemodynamic state. A voltage proportional to this pressure can be obtained via a piezo-electric device affixed to the end of a permanently implanted transvenous and intracardiac lead. In such a device a pressure sensor acts as one arm of a resistive bridge and varies its resistance, and therefore the voltage across it, with the pressure applied to it. A voltage waveform can thereby be obtained that reflects the changes in ventricular pressure, and therefore haemodynamnic state, over time.
With respect to bradycardia support pacing, one of the common strategies of optimizing cardiac output for a patient is to alter the A-V delay and/or pacing rate of his pacemaker. The latter in particular is fraught with risk. The patient must be carefully monitored after such manipulations since the patient may be pushed into heart failure.
Wish et al. ("Importance of Left Atrial Timing in the Programming of Dual-Chamber Pacemakers," Am. J. Cardiol 60: 566-571 (1987)) have shown that stroke volume can be optimized by manipulating the A-V delay. The optimal value for the A-V delay varies from patient to patient and with the pacing mode used. The present strategy is to use electrophysiological studies to determine the best value of A-V delay. However such studies are not without risk and must be repeated as a patient's clinical status varies over time.
Mirowski et al., in U.S. Pat. Nos. 3,614,955 and 3,942,536 describe systems that sense heart function using the peak of the right ventricular pressure waveform. Devices of this kind, which have yet to be commercially implemented, suffer some obvious disadvantages.
Generally devices which monitor only the peak of the ventricular pressure waveform are unable to initiate antitachycardia pacing and to automatically optimize bradycardia support pacing. Additionally, such devices are designed to use the raw RVP waveform. The common implementation of such a device utilizes a piezo-electric transducer. If such a device is implemented, it suffers the wandering baseline associated with piezo-electric pressure transducers and an inability to respond to alterations in a given patient's degree of right/left sided heart failure and pulmonary hypertension.
In prior art devices, the use of a pressure reference in a pressure sensing lead produces problems when the devices are intended to be permanently implanted. As noted above, typical piezo-electric sensors suffer from baseline drift. This results in a variable direct current offset being added to the ventricular pressure waveform even when a pressure reference is built into the device.
A device disclosed in U.S. Pat. No. 4,774,950 to Cohen seeks to overcome the shortcomings of the common forms of pacemakers by relying on the mean RVP, mean arterial pressure, mean left atrial pressure, mean left ventricular pressure and/or mean central venous pressure as indicators of haemodynamic compromise. The background to this invention can be found in Cohen et al.'s article "Haemodynamic Responses to Rapid Pacing: A Model for Tachycardia Differentiation.", PACE 11: 1522-1528 (1988).
The Cohen patent discloses a device that either uses discrete circuitry or a microprocessor to perform its functions.
The use of a microprocessor in a pacemaker is not uncommon. However the manipulations described are expensive in the use of both power and microprocessor cycle time when implemented in an implantable device. There are simpler measures of haemodynamic compromise that can be used.
It is well recognized that atrio-ventricular (AV) synchrony, cardiac rate and cardiac ejection volume interact to determine cardiac output. In this regard, reference is made to the article by B. N. Goldreyer, "Physiologic Pacing: The Role of AV Synchrony." PACE 5: 613-615 (1982). Of these, the two former are open to manipulation by a bradycardia support pacemaker. A disadvantage of present programmable devices is that they must be reprogrammed should the recipient's condition change. This involves the expense and inconvenience of a visit to a hospital and drastically reduces the ability of the device to respond to changes in the recipient's condition.
It would thus be advantageous for a pacemaker to have the ability to manipulate automatically these and other pacing parameters to guarantee the best possible bradycardia pacing effect.